The present invention relates to an electrostatic developer and an electrostatic developing method for use in electrophotography, electrostatic recording or the like.
Heretofore, as a developing method and a developer for use in electrostatic development in electrophotography or the like, there have been known, for example, a two-component developing method by using as a developer a mixture of nonmagnetic image-forming particles and magnetic particles having a larger particle size than that of the image-forming particles, a one-component developing method by using a developer comprising only magnetic image-forming particles or magnetic image-forming particles and a small amount of additive particles, and a developing method by using a developer comprising magnetic particles having a larger particle size than that of the image-forming particles and the above-mentioned one-component developer.
In the said magnetic one-component developing method and developing method of using as a developer the mixture of the magnetic image-forming particles and magnetic particles of the larger particle size than that of the image-forming particles, there has been known a method of moving a permanent magnet in which N poles and S poles are disposed alternately at the back face of a developer-carrying member, thereby delivering the developer and at the same time, periodically decreasing the magnetic attraction force of the image-forming particles to the direction of the carrying member under an alternating (oscillating) magnetic field caused by the movement of the permanent magnet in a region opposed to an electrostatic latent image-retaining member, thereby improving the developability to the latent image-retaining member (for example, refer to Japanese Patent Application Laid-Open (KOKAI) Nos. 50-45639(1975) and 58-100869(1983)); a method of reciprocally moving charged image-forming particles under an alternating (oscillating) electric field formed by an AC voltage applied between a latent image-retaining member and a developer-carrying member (for example, refer to U.S. Pat. No. 3,866,574 and Japanese Patent Application Laid-Open (KOKAI) No. 55-18656(1980)); or a method in which both of the systems are used together.
However, the developing method under the alternating electric field involves a drawback of also promoting the generation of oppositely charged image-forming particles and transfer of the oppositely charged image-forming particles to a non-image area of a latent image-retaining member. In the alternating electric field, since the developer tends to be exposed to a stronger electric field as compared with that under a static electric field, charge transfer between the image-forming particles may be promoted to generate oppositely charged image-forming particles.
Further, upon development under the alternating electric field or alternating magnetic field, since the attracting force to the direction of the developer-carrying member is reduced, oppositely charged image-forming particles which are charged relatively weakly and which can not transfer upon development under a static field or static magnetic field, are also transferred easily. Transfer of the image-forming particles to the non-image area causes to an undesirable phenomenon in view of image quality as a so-called "fogging", in which black spots are formed in the white area of images. In a copying machine in which developed image-forming particles on a latent image-retaining member are electrostatically transferred to a transfer material such as paper and images are finally visualized, the image-forming particles in the non-image area are not sometimes transferred to the transfer material because they are charged oppositely. In such case, since more image-forming particles than required are consumed, this is an undesirable phenomenon.
Further, imbalance occurs between the front and rear ends of images depending on the moving direction of a developing sleeve or magnet. According to the result of an experiment made by the present inventors, in a usual magnetic one-component developing method in which the developing sleeve is moved in the same direction as movement of the latent image-retaining member while the magnet is moved in a reverse direction to movement of the latent image-retaining member in an opposed region between the latent image-retaining member and the developing sleeve (developer-carrying member), "underdevelopment of front edge" in which uniform image density is difficult to be formed and front ends of solid images are disturbed, or "overdevelopment of rear edge" in which rear ends of the solid images are excessively developed is caused. In addition, also in the development of fine line images, image-forming particles scatter around the periphery of fine lines, whereby the development only with low resolution power and low image quality arises.
As a result of the present inventors' earnest studies for overcoming the foregoing problems, it has been found that by using a developer composed of conductive particles having a specific average particle size and particles having a specific charge property and subjected to a specific surface treatment, a development at high resolution power with no transfer of oppositely charged image-forming particles to a non-image area, with high density and clear images and without the unnecessary consumption of image-forming particles in electrophotography, electrostatic recording or the like is attained. The present invention has been accomplished based on the finding.